In principle, electrical loads and load elements can be differentiated into so-called NTC (negative temperature coefficient) and PTC (positive temperature coefficient) thermistors. NTC thermistors are distinguished by a negative temperature coefficient, while PTC thermistors have a positive temperature coefficient. This means that in the case of NTC thermistors, such as in the case of semiconductors, for example, a temperature increase brings about a decrease in the electrical resistance. By contrast, in the case of PTC thermistors, a temperature increase also leads to a higher electrical resistance.
Many loads such as, for example, incandescent lamps or else other loads which have a PTC thermistor characteristic are increasingly being driven by an integrated switching unit. Integrated switching units have the advantage that they enable an electronic control in comparison with conventional switching units, so that various additional functions can be realized. In many cases it is also possible to reduce the current consumption, save space or, for example, monitor a load more simply.
Semiconductor components or integrated microelectronic circuits are generally used for an integrated switching unit. What is disadvantageous about semiconductor components and integrated semiconductor circuits, in principle, is that they can be destroyed comparatively easily by excessively high operating parameters such as, for example, voltage, current, electrical charge, temperature and air humidity. This concerns the reliability, on the one hand, in that the components are destroyed at elevated operating parameters over the course of time or else as a result of limit values of individual operation parameters being exceeded once. With the use of integrated semiconductor circuits for electrical loads, overload situations particularly often lead to a destruction of the switching unit. Situations of this type may occur for example as a result of short circuits or else as a result of switch-on or switch-off operations which cause high short-circuit currents and hence overloads.
There are various strategies for avoiding destruction of a semiconductor switching unit. The most widespread strategy consists in performing a temperature-dependent switch-off in the case of an excessively high energy dissipation, particularly at high ambient temperatures. What is disadvantageous about this method is a persistently high temperature, which has a disadvantageous effect on the reliability, and may finally also lead to a destruction of the switching unit. Other strategies employ linear current limiting that keeps the electric current in the switching unit below predetermined limit values. However, particularly in the case of the switch-on operation, in the case of capacitive loads or else in the case of incandescent lamps, this method leads to an increased voltage drop accompanied by a high power loss and thus a rapid temperature increase. The high temperature and, in particular, also the great temperature changes adversely influence the reliability of the switching unit, so that the latter may likewise be destroyed over the course of time.
A further strategy consists in performing a switch-off in a manner dependent on the electric current with a number of current levels adapted to the load. In the case where a respective current level is exceeded, the switching unit is immediately deactivated in order to protect it against destruction. However, this in turn has the disadvantage that when the switching unit is subjected to a restart initiated in response to this, it must be taken into account that the temperature of the load is still increased and the limiting current level may therefore be too high. This may in turn carry an excessively high overload during the switch-on operation in the switching unit, which may result in destruction of the switching unit.